Image sensor with high dynamic range and method

ABSTRACT

In one form, a pixel for use in image sensing comprises a photodetector, a sink device, and a readout circuit. The photodetector is formed in a semiconductor substrate and has a charge collection region for receiving photocharge representative of incident light. The sink device is formed in the semiconductor substrate and adjacent to the charge collection region and has a gate overlying and insulated from the semiconductor substrate and receiving a responsivity control signal. The readout circuit transfers the photocharge collected by the charge collection region of the photodetector to an output in response to a select signal. In another form, the pixel may be used in an image sensor having a pixel array of such pixels.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to image sensors, and moreparticularly to image sensors with high dynamic range.

BACKGROUND

Electronic image sensors are found in a variety of useful products,including cameras, camcorders, cell phones, medical devices, machinevision instruments, and the like. Image sensors have a characteristicdynamic range. Dynamic range refers to the range of incident light thatcan be accommodated by an image sensor in a single frame of pixel data.It is desirable to have an image sensor with a high dynamic range toimage scenes that generate high dynamic range incident signals, such asindoor rooms with windows to the outside, outdoor scenes with mixedshadows and bright sunshine, night-time scenes combining artificiallighting and shadows, and many others.

For example in cameras there are generally two ways to adjust imageexposure to achieve high dynamic range. The first is to change theshutter speed. The second is to change the size of the aperture. Both ofthese ways of adjusting image exposure control the quantity of lightthat is applied to film or to an electronic image sensor. In digitalcameras, the image sensor can change the shutter speed electronically.However the use of pulsed light emitting diode (LED) light has becomecommon, and keeping sensor integration time constant is even moreimportant because of possible flickering effect related to shortintegration time. Moreover changing the lens aperture is not verypractical for machine vision applications.

There are several known techniques for extending the dynamic range ofimage sensor pixels themselves, including the use of companding pixels,logarithmic pixels, dual conversion gain pixels, and dual photodiodepixels. However each of these approaches has drawbacks. A drawback ofcompanding pixels is their nonlinear response, high pixel fixed patternnoise (FPN), and drop in signal-to-noise ratio (SNR) at knee points. Thelogarithmic pixel is also subject to high pixel FPN and nonlinearity.Dual conversion gain and dual photodiode pixels have only twopredetermined, discrete responsivity values, which limit theirusefulness.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings, in which:

FIG. 1 illustrates in schematic form an active pixel known in the priorart;

FIG. 2 illustrates in block diagram form an image processing system withhigh dynamic range according to the present invention;

FIG. 3 illustrates in block diagram form the image sensor used in theimage processing system of FIG. 2;

FIG. 4 illustrates a cross section of a pixel with an associated sourcefollower transistor used in the image sensor of FIG. 3;

FIG. 5 illustrates the cross section of the pixel of FIG. 4 with acorresponding potential well diagram under a first bias condition;

FIG. 6 illustrates the cross section of the pixel of FIG. 4 with acorresponding potential well diagram under a second bias condition; and

FIG. 7 illustrates the cross section of the pixel of FIG. 4 with acorresponding potential well diagram under a third bias condition.

The use of the same reference symbols in different drawings indicatessimilar or identical items. Unless otherwise noted, the word “coupled”and its associated verb forms include both direct connection andindirect electrical connection by means known in the art, and unlessotherwise noted any description of direct connection implies alternateembodiments using suitable forms of indirect electrical connection aswell.

DETAILED DESCRIPTION

FIG. 1 illustrates in schematic form a pixel 100 known in the prior art.Pixel 100 is an active pixel that includes a photodiode 110, a transfergate 120, a floating diffusion 130, N-channel metal-oxide-semiconductor(MOS) transistors 140, 150, and 160, and a column conductor 170.Photodiode 110 has a cathode, and an anode connected to ground.Transistor 120 has a drain connected to a floating diffusion (FD) 130, agate for receiving a control signal labeled “TG”, and a source connectedto the cathode of photodiode 110. Transistor 140 has a drain connectedto a power supply voltage terminal labeled “V_(AA)”, a gate forreceiving a signal labeled “RST”, and a source connected to floatingdiffusion 130. Transistor 150 has a drain connected to V_(AA), a gateconnected to floating diffusion 130, and a source. Transistor 160 has adrain connected to the source of transistor 150, a gate for receiving asignal labeled “RS”, and a source connected to column conductor 170.

Signal RS is a row select signal that activates pixel 100 and all pixelswithin the same row of the image sensor. In a typical implementation,the image sensor uses correlated double sampling (CDS) in which theacquired image level is subtracted from the sampled reset level tocompensate for fixed pattern noise. During a reset period, pixel 100 isreset in response to the activation of signal RST, which makestransistor 140 conductive and pulls up floating diffusion 130 to arelatively high voltage. During the reset period, transfer gate signalTG is inactive. Transistor 150 provides a voltage on its sourcecorresponding to the voltage on floating diffusion 130 minus thethreshold voltage of transistor 150. Since signal RS is active,transistor 160 is conductive and passes the reset level to columnconductor 170.

During image acquisition, signal RST is inactive and charge carriers (inthis case electrons) accumulate at the cathode of photodiode 110 in anamount corresponding to incident light. Signal TG is activated totransfer the accumulated electrons to floating diffusion 130, reducingthe voltage on the floating diffusion. Transistor 150 is connected in asource follower configuration and acts as an active amplifier to bufferthe voltage on the floating diffusion to provide a voltage on its sourceequal to V_(FD)−V_(T), in which V_(FD) is the voltage of the floatingdiffusion and V_(T) is the threshold voltage of transistor 150. Sincetransistor 160 is conductive, this voltage is transferred to columnconductor 170.

FIG. 2 illustrates in block diagram form an image processing system 200with high dynamic range according to the present invention. Imageprocessing system 200 may be, for example, a digital still camera, acell phone camera, a digital video camcorder, and the like. Imageprocessing system 200 includes generally an imaging stage 220, an imagesensor 230, a processor 240, a memory 250, a display 260, and otherinput/output (I/O) devices 270.

In image processing system 200, imaging stage 220 receives light 210from a subject scene. Imaging stage 220 can include conventionalelements such as a lens, a neutral density filter, an iris, and ashutter. Imaging stage 220 focuses light 210 to form an image on imagesensor 230. Image sensor 230 captures images by converting the incidentlight into electrical signals. Processor 240 then performs postprocessing on the images so acquired and converts them into variousformats for output on display 260 or export using other I/O devices 270.Processor 240 may be implemented, for example, with a microprocessor,microcontroller, digital signal processor (DSP), or other digital logiccircuit, and processor 240 also provides signals to control variouselements of image processing system 200. Processor 240 uses memory 250to store acquired images for presentation on display 260. Display 260may be any type of display, such as an active matrix color liquidcrystal display (LCD). The other I/O devices 270 may include, forexample, various on-screen controls, buttons or other user interfaces,network interfaces, memory card interfaces, and the like.

FIG. 3 illustrates in block diagram form the image sensor 230 used inimage processing system 200 of FIG. 2. Image sensor 230 includesgenerally a pixel array 310, a timing and control circuit 320, a decoder330, a row driver 332, a decoder 340, a column select circuit 342, asample-and-hold amplifier labeled “S/H” 350, an amplifier 360, ananalog-to-digital converter (ADC) 370, and an image processor 380. Pixelarray 310 includes an array of pixels arranged in rows and columns.Timing and control circuit 320 controls decoder 330 to activate eachselected row in sequence. Decoder 330 selects a single row line, and rowdriver 332 drives a corresponding row select signal across an entire rowof pixel array 310. Each pixel along the selected row is activated atthe same time by the common row select line, and provides an output to arespective column line.

In the illustrated embodiment, timing and control circuit 320 alsoselects the columns in sequence by providing a column address to decoder340. Decoder 340 provides a column select signal to column selectcircuit 342. Column select circuit 342 connects the column line tosample-and-hold amplifier 350. Sample-and-hold amplifier 350 providesboth a reset level labeled “V_(RST)” and a signal level labeled“V_(SIG)” to amplifier 360. Amplifier 360 is a voltage amplifier whichsubtracts the signal level from the reset level to compensate for fixedpattern noise. Analog-to-digital converter 370 receives the differencevoltage and provides a digital code representative of thenoise-compensated signal level of the pixel to image processor 380,which accumulates to the signals to form an entire image. Imageprocessor 380 performs additional image processing functions to providea processed image signal labeled “OUTPUT”, and provides an output signallabeled “ILLUMINATION LEVEL” to timing and control circuit 320 torepresent the light intensity of the whole image or certain portionsthereof.

In addition, timing and control circuit 320 provides a specialresponsivity control signal labeled “RC” to pixel array 310. Timing andcontrol circuit 320 uses signal RC to adjust the responsivity of pixelsin pixel array 310 to keep selected pixels from saturating and therebyimprove dynamic range. In one form, timing and control circuit 320provides signal RC based on the intensity of light of the whole image.In this example, timing and control circuit 320 operates as a variablevoltage source that provides the RC signal in response to theILLUMINATION LEVEL. In another form, timing and control circuit 320provides signal RC at different levels based on the intensity of lightof particular portions of the image, such as on a row-, column-, orregion-basis. Each pixel in pixel array 310 is modified in a manner tobe described below to use the value of the RC signal to adjust thesaturation level of the pixel, thus providing a wider overall dynamicrange.

FIG. 4 illustrates a cross section of a pixel 400 with an associatedsource follower transistor 450 used in image sensor 230 of FIG. 3. Pixel400 is formed in a semiconductor substrate 410 having a front surface412 and a back surface 414. Pixel 400 includes generally a photodioderegion 420, a responsivity control gate region 430, a readout circuitregion 440, and source follower transistor 450.

Photodiode region 420 includes a buried n+ cathode region 422 underlyinga p+ isolation region 424 in a surface portion of semiconductorsubstrate 410 near front surface 412 to form a pinned photodiode.

Responsivity control gate region 430 includes a gate 432 isolated fromfront surface 412 by a thin layer of gate oxide, and an n+ drain region434. Gate 432 and drain region 434 are connected together and receivesignal RC, thus forming a diode-connected MOS transistor.

Readout circuit region 440 includes a transfer gate 442, an n+ floatingdiffusion 444, a reset gate 446, and an n+ drain region 448 connected topower supply voltage terminal V_(AA). Each of gates 442 and 446 isisolated from front surface 412 by a thin layer of gate oxide and isused to induce a conductive channel for conducting electrons in responseto the application of a positive voltage above the threshold voltages ofthe respective transistors.

Transistor 450 has a drain connected to V_(AA), a gate connected tofloating diffusion region 444, and a source connected to node 452, whichitself is further connected to a drain of a row select transistor, notshown in FIG. 4.

Pixel 400 operates as described with respect to pixel 100 of FIG. 1except that it includes an additional sink device in the form of a MOSdiode-connected transistor. Gate 432 selectively forms a conductivechannel between the pinned photodiode and drain region 434 based on thevoltage of signal RC. Drain region 434 collects photocurrent based onthe potential of signal RC. When the potential under gate 432 is low, nodepletion layer will form and most photocurrent formed by backsideincident light will be collected by cathode 422. The depletion layerunder gate 432 is always in a non-stationary condition because n+ drainregion 434 prevents mobile inversion charge build-up under gate 432. Allphoto electrons attracted by gate 432 are swept into the virtual powersupply driving the RC node, helping to create a deep depletion region.Surface potential under gate 432 is given by:

$\begin{matrix}{V_{RC} = {V_{FB} = {\phi_{s} + \frac{\sqrt{2ɛ_{s}{qN}_{A}\phi_{s}}}{C_{ox}}}}} & \lbrack 1\rbrack\end{matrix}$

in which V_(RC) is the voltage of gate 432, V_(FB) is the flat bandvoltage, φ_(s) is the surface potential, ∈_(s) is the silicon dielectricpermittivity, q is the electron charge, N_(A) is the substrate dopingconcentration, C_(ox) is the oxide capacitance. The pixel photocurrentI_(ph) can be expressed by:

I _(ph) =I _(PD) +I _(RC)  [2]

in which I_(PD) is the photodiode current and I_(RC) is the RC gatecurrent. The ratio of these two currents can be estimated by:

$\begin{matrix}{\left. \frac{I_{PD}}{I_{RC}} \right.\sim\frac{W_{PD}}{W_{RC}}} & \lbrack 3\rbrack\end{matrix}$

in which W_(PD) is the volume of the PD depletion region and W_(RC) isthe volume of the RC gate depletion region.

A pixel with a sink device such as a diode-connected MOS transistorcontrolled by a responsivity control gate provides the ability tocontrol photodiode current and thus maximum light intensity causingsaturation of the pixel. This feature is useful for high dynamic rangeand machine vision applications, especially when pulsing LED lightprohibits using short integration time. Using a diode-connectedtransistor biased to a virtual supply node helps with crosstalkreduction. In addition, the responsivity control gate can also be usedfor white balance purposes.

FIG. 5 illustrates the cross section of pixel 400 of FIG. 4 with acorresponding potential well diagram 500 under a first bias condition.The first bias condition corresponds to signal V_(RC) at a lowpotential. A potential well 520 is formed by the cathode of photodiode420, and causes photocharge to build up over the integration period. Thelow value of V_(RC) forms a significant potential barrier such thatsubstantially no electrons are able to reach drain region 434 andsubstantially all the accumulated photocharge remains in potential well520 under cathode 422.

FIG. 6 illustrates the cross section of pixel 400 of FIG. 4 with acorresponding potential well diagram 600 under a second bias condition.The second bias condition corresponds to signal V_(RC) at a mediumpotential. A potential well 620 is again formed by the cathode ofphotodiode 420, and causes photocharge to build up over the integrationperiod. The medium voltage of V_(RC) forms a small potential well 632under gate 432 and some electrons are able to reach drain region 434.Thus at medium potential bias, pixel 400 saturates at a higherillumination level that at low bias.

FIG. 7 illustrates the cross section of the pixel of FIG. 4 with acorresponding potential well diagram 700 under a third bias condition.The third bias condition corresponds to signal V_(RC) at a highpotential. A potential well 720 is again formed by the cathode ofphotodiode 420, and causes photocharge to build up over the integrationperiod. The high voltage of V_(RC) forms a large potential well 732under gate 432 and many electrons are able to reach drain region 434.Thus at medium potential bias, pixel 400 saturates at a higherillumination level than at medium bias.

The saturation level increases monotonically with increasing V_(RC)voltage. The increase need not be linear and timing and control circuit320 can select the bias voltage V_(RC) from a lookup table to correspondto the desired saturation level.

Thus, a pixel with adjustable responsivity includes a sink device, suchas a diode-connected MOS transistor, that can be dynamically biased tocontrol the responsivity and hence the intrascene dynamic range of theimage sensor. An image array formed by such pixels is responsive to oneor more responsivity control signals to increase or decrease theresponsivity of the array or of certain portions of the array based onthe ILLUMINATION LEVEL of the scene.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments that fall within thetrue scope of the claims. For example in other embodiments, theconductivity type of the semiconductor substrate, photodiode, andtransistors can be reversed Moreover the image sensor and imageprocessing system can be used to increase the dynamic range of a varietyof different electronic products.

Thus, to the maximum extent allowed by law, the scope of the presentinvention is to be determined by the broadest permissible interpretationof the following claims and their equivalents, and shall not berestricted or limited by the foregoing detailed description.

What is claimed is:
 1. An image sensor comprising a pixel array, eachpixel of said pixel array comprising: a photodetector formed in asemiconductor substrate and having a charge collection region forreceiving photocharge representative of incident light; a sink deviceformed in said semiconductor substrate and adjacent to said chargecollection region having a gate overlying and insulated from saidsemiconductor substrate and receiving a responsivity control signal; anda readout circuit for transferring said photocharge collected by saidcharge collection region of said photodetector to an output in responseto a select signal.
 2. The image sensor of claim 1 wherein said sinkdevice further comprises a drain for receiving said responsivity controlsignal.
 3. The image sensor of claim 1 wherein said sink device is on afirst side of said charge collection region and said readout circuit ison a second side of said charge collection region opposite said firstside.
 4. The image sensor of claim 1 further comprising: a variablevoltage source for providing said responsivity control signal inresponse to a measured illumination level.
 5. The image sensor of claim1 wherein: said incident light comprises light incident upon a backsurface of said semiconductor substrate.
 6. The image sensor of claim 1wherein: said photodetector comprises a buried photodiode.
 7. The imagesensor of claim 6 wherein: said buried photodiode comprises a pinnedphotodiode.
 8. The image sensor of claim 1 wherein said readout circuitcomprises: a transfer gate adjacent to said photodetector and formed ina front surface of said semiconductor substrate and insulated from saidfront side of said semiconductor substrate, for forming a conductivechannel from said charge collection region to a floating diffusion; anda source follower transistor having a drain coupled to a power supplyvoltage terminal, a gate coupled to said floating diffusion, and asource for providing a pixel output signal.
 9. A pixel for use in imagesensing comprising: a photodetector having a charge collection regionfor receiving photocharge representative of incident light; a sinkdevice having a source coupled to said charge collection region, a gatefor receiving a responsivity control signal, and a drain; and a readoutcircuit having an input coupled to said charge collection region, and anoutput coupled to an output conductor for providing a pixel outputthereto.
 10. The pixel of claim 9 wherein said drain of said sink deviceis coupled to said gate thereof.
 11. The pixel of claim 9 wherein saidsink device is on a first side of said charge collection region and saidreadout circuit is on a second side of said charge collection regionopposite said first side.
 12. The pixel of claim 9 wherein: saidphotodetector comprises a buried photodiode.
 13. The pixel of claim 12wherein: said buried photodiode comprises a pinned photodiode.
 14. Thepixel of claim 9 wherein said readout circuit comprises: a transfer gatefor transferring remaining charge from said charge collection region toa floating diffusion; and a source follower transistor having a draincoupled to a power supply voltage terminal, a gate coupled to saidfloating diffusion, and a source for providing a pixel output signal.15. The pixel of claim 14 wherein said readout circuit furthercomprises: a reset gate for coupling a reference voltage terminal tosaid floating diffusion in response to an activation of a reset signal.16. A method for converting incident light into an electrical signal,comprising: collecting charge in a charge collection region in responseto said incident light; diverting a portion of said charge to a drain inresponse to a level of a responsivity control signal; and reading outremaining charge from said charge collection region.
 17. The method ofclaim 16 wherein said diverting comprises: biasing said chargecollection region using said responsivity control signal.
 18. The methodof claim 16 wherein said reading out comprises: transferring saidremaining charge to a floating diffusion; and providing a voltage on anoutput line in response to said charge in said floating diffusion. 19.The method of claim 18 wherein said reading out further comprises:resetting a voltage of said floating to a predetermined voltage prior tosaid diverting.
 20. The method of claim 19, wherein said reading outfurther comprises: sampling a voltage on said floating diffusion aftersaid resetting; sampling said voltage on said floating diffusion afterreading out said remaining charge; and subtracting said voltage on saidfloating diffusion after said resetting from said voltage on saidfloating diffusion after reading out said remaining charge to form apixel output signal.